Several methods exist for management of mountain beetle population but few effectively prevent damage of high value pine trees, for example, in campgrounds near administrative sites, near summer homes and wildlife refuges, and in riparian zones. One method of management of the mountain pine beetle is to harvest infested trees and kill the beetles resident in the inner bark during milling. This method often fails because it is not feasible or desirable to harvest the pine trees before the beetles emerge and disperse to new trees; also, many stands of trees are inacceptible and harvesting of all infested or high risk stands is not compatible with management of other resource values.
Use of conventional chemical pesticides against the beetles is also of limited utility because the beetles spend most of their life protected under the bark of infested trees and thus do not come in lethal contact with the pesticides.
A new method of management of beetles is the use of semiochemicals (message-bearing chemicals). The mountain pine beetle produces and releases several attractive semiochemicals during tree colonization. Several of these chemicals are attractive to other beetles and serve to initiate mass attack on host trees (Borden et al. 1987). During initial and mass attack, the host tree releases extraordinarily high quantities of pitch which contain terpenes, some of which are attractive to the beetles. One of these terpenes, myrcene (7-methyl-3-methylene-1,6-octadiene) synergizes attraction of beetles to two beetle-produced attractants trans-verbenol (trans-4,6,6-trimethylbicyclo[3.1.1]hept-3-en-2-ol) and exo-brevicomin (exo-7-ethyl-5-methyl-6,8-dioxabicyclo-[3.2.1]octane) (Borden et al. 1983, 1986, 1987; Conn et al. 1983). A beetle attractive mixture of myrcene, trans-verbenol and exo-brevicomin is currently used in North America to bait trees on which it is desired to induce attack (Borden et al. 1986).
There are several current strategies for combatting the mountain pine beetle using these attractive semiochemicals. One involves attraction to baited trees within an infested area, to effect containment and concentration of the infestation prior to removal of the beetles by logging. A second involves attraction to baited trees in which the beetles will be killed by injection of a chemical pesticide, or treating the outside bark with lethal pesticides.
In some instances it is more desirable to repel the beetles from trees or stands by use of a repellant than it is to lure beetles to trees for the purpose of destroying both the tree and attracted beetles. This approach has been investigated using pine oil obtained from the pulping process but due to the high cost of experiment and registration, and possibly for its limited use in high value oils, further testing is not recommended at this time.
The negative enantiomer of verbenone (4,6,6-trimethylbicyclo[3.1.1]hept-3-en-2-one) has been shown to inhibit response of both sexes of the mountain pine beetle to the attractive volatiles, a-pinene, trans-verbenol and myrcene in laboratory bioassays, as well as in field trap bioassays (Ryker and Yandell, 1983). Verbenone has been shown to inhibit male beetle response to multiple funnel traps baited with attractive mixtures of trans- verbenol, exo-brevicomin and myrcene but it did not significantly alter response of female beetles to these baited traps (Borden et al. 1987). Vite (U.S. Pat. No. 4,839,383) discloses antiaggregant activity for the negative enantiomers of verbenone, endo-brevicomin and exo-brevicomin, on the southern pine beetle, Dendroctonus frontalis. His data cannot be extrapolated to other species of the genus Dendroctonus. Species within a genus may respond differently to the same compound, or its enantiomers. For example, the ambrosia beetle Gnathotrichus sulcatus is attracted by racemic sulcatol (6-methyl-5-hepten-2-ol), while G. retusus, a sympatric species, is attracted by S-(+)-sulcatol, while even as little as 5% of the R-(-)-enantiomer affects its response negatively (Borden et al. 1980). Lanier and Wood (1975) demonstrated that interspecific attraction to pheromones occurs mainly within the same species group in the genus lps. They also demonstrated differences in attraction between two subspecies of l. calligraphus, and between eastern and western populations of l. pini. This shows that extrapolations of data from one species to another cannot always be made.
Several instances of repulsion of bark beetles by their own attractive chemical emissions have been documented. On the basis of trapping experiments, exo-brevicomin and endo-brevicomin (endo-7-ethyl-5-methyl-6,8-dioxabicyclo[3.2.1.]octane have both been reported to exhibit repellant behaviour toward mountain pine beetle (Rudinsky et al. 1974a), yet exo-brevicomin is part of the presently used operational attractive bait used to lure these insects to baited trees. Frontalin (1,6-dimethyl-6,8-dioxabicyclo[3.2.1]octane) is produced by feeding male beetles and is reported to have a repellant effect in beetle trapping experiments (Libbey et al. 1985). However, frontalin has also been shown to induce attack on pine by the mountain pine beetle (Chatelain and Schenk, 1984) and has been shown to be attractive in one of two trapping experiments by Borden et al. (1987). Conversely, E-myrcenol has been shown to repel another bark beetle, Ips pini, from traps, but to induce attack on host logs (D. R. Miller, Unpublished).
3-Methyl-2-cyclo-hexen-1-one (MCH) has been shown to serve as an anti-aggregation pheromone for the Douglas-fir beetle, Dendroctonus pseudotsugae, and the spruce beetle, D. rufipennis (Rudinsky et al 1974b). An aerially applied bead formulation of MCH for protecting felled Douglas-fir trees (Furniss et al. 1981, 1982; McGregor et al. 1984) has been protected in U.S. Pat. No. 4,170,631 (Young, R. W. and M. M. Furniss, Oct. 9, 1979). The use of MCH formulated in bubble gaps and applied to the trunk of individual, felled Douglas-fir trees and spruce trees were demonstrated to be efficient for protection against attacks by Douglas-fir beetle and spruce beetle, respectively (Lindgren et al. 1988; 1989). Based on this knowledge, it is not possible to predict or infer the response of the mountain pine beetle to pine trees or pine forests treated with verbenone.